An organic electroluminescence flat display has many significant advantages, such as initiative light-emitting, light, thin, good contrast, independence of an angle, low power consumption and the like. In 1963, an organic electroluminescence device was fabricated by Pope et al with an anthracene single crystal. However, the first high efficient organic light-emitting diode (OLED) fabricated by vacuum evaporation was an OLED developed by C. W. Tang et al in 1987, wherein aniline-TPD was used as a hole transport layer (HTL), and a complex of aluminium and 8-hydroxyquinoline-ALQ was used as a light-emitting layer (EML). Its operating voltage was less than 10V, and its luminance was up to 1000 cd/m2. The light-emitting wavelength of organic electroluminescence materials developed later could cover the whole range of visible light. This breakthrough development made the field becoming a currently research hotspot. After entering 1990s, organic high molecular optical-electric functional materials entered a new development stage.
In an organic electroluminescence device, in order to reduce the operating voltage of the device and improve the charge balance between electrons and holes, it is necessary to increase electron injection efficiency.
The use of a low work function metal as a cathode can effectively improve electron injection ability. However, the low work function metal is too active and apt to react with water and oxygen.
Another method for improving electron injection ability is to add an electron injection layer formed of an inorganic compound layer between said cathode and an organic layer. It has been proved in practice that LiF/Al is a cathode structure having excellent electron injection ability, which is widely used in OLED products. However, the present of halogen elements may cause quench of light emission, materials exhibit relatively large poisonousness, and the film-forming temperature of the materials is high. Moreover, the thickness of the formed electron injection layer is critical; and when Li2O, LiAlO2 or Li2CO3 is used as a injection layer material, the lifetime is short and the efficiency is low.